How to Dye Acrylic black?

[ImanCarrot]

Anyone got any ideas how to please?

At the moment I'm using Dye from BPI, which is used to tint lenses, it dyes my blanks fine, no problem, but when I go to cut it, it fractures.

I'm making a small hole in a 25mm diameter dyed lens- the black is to stop stray reflections, the hole lets the light through.

The dye I use works at 95 deg C and takes about 1/2 a day to blacken the components, I pre- prepare them in a 60 deg C oven and cool them down gradualy to eliminate thermal shock.

I've wasted 3 days on this now and am staring perplexedly at lots of fractured scrap components.

Is there another way? *pulls hair out*

[wildwestpat]

Acrylic is available in opaque black and various neutral density filter material by ND value. This would avoid coating the material - but - why not coat after machining? One source of supply in UK is http://www.plasticonline.co.uk/index...wCat&catId=159



Regards Pat

[MadMax]

Can you switch to polycarbonate? Unfortunately both materials are quite sensitive to oils and solvents (polycarb less so). Exposure many types of oils or solvents can cause micro cracking (crazing) of these materials. In fact you want to be careful what coolants you use in your machining operations. Make sure you're not shooting tramp oil emulsion on your acrylic parts.

[MrWild]

The blank fractures,or the dye coating does? If a coating, can you mask the lense some way?

[ImanCarrot]

Thanks for the responses.

I didn't explain well enough- I have to machine a "hole" onlly 10um deep, not all the way through, then a lens on the other surface so ND filters are out.

Can't coat after machining, the optical surfaces need to be clear. And I can't paint it the componentts get cemented together- the paint would add thickness, there's 13 components to be cemented together- paint would throw the tolerance out.

Can't switch to polycarb, the refractive index is wrong for the optical design.

Am using spraymist water and methanol.

It's the actual substrate that crazes all the way through.

Can't mask the clear area- it's only 1mm in diameter.


Thanks for the kind replies though.

[cpcp]

Couple of thoughts:

Presume you have spoken to BPI

Drop an uncoated chip in your coolant for 12-24hrs and see if it has started to craze; I never use methanol/ethanol it seems to find a way into even the best annealed acrylic for me (I prefer compressed air but that may be difficult on such small tooling).

Perhaps though the crazing is already there before you start to machine in which case (ruling out the dye) perhaps you ramped up your temperatures too fast or got thermal shock from the 60 to 90 change.

The other possibility is the quality and age of the stock. There is more and more rubbish about purporting to be Cast Acrylic which is really continuously cast off machines like other plastic sheet but to do this they have to use more plasticizer making it softer and more porous. Also stock that hasn't had proper UV additives and has been sitting in the sun or temperature cycles can become subject to brittleness and cracking. Try a sample from a new supplier.

Regarding the masking ideas have you thought about using a tiny drop of wax that melts at say 110 degrees?

[ImanCarrot]

Good thoughts Cpcp, thank you.

[ImanCarrot]

Phoned the customer... apparently it's only the central region that's used- crazing at the edges doesn't matter *bangs head on monitor*

[cpcp]

Haha what a pain.

BTW didn't realize it was just the edges; thought that the crazing was happening/discovered as you milled into the middle. If its the edges then I suspect that they were cut pretty hard inducing some hairline fracture weakness that the cutting fluid entered and aggravated. If you annealed before dyeing you may have had a different outcome.

Glad it worked out.

[Signmaker]

Iman,

I bet you it's the methanol which is causing all your problems. We NEVER clean any of our plastics with methanol.... it's way, way to harsh. If you are seeing lots of small cracks and silvering this is a classic sign of chemical attack.

You might want to switch to fairy liquid... we use this a lot for lubricating acrylic when it's been cut. Alternatively you might want to switch to a product called Glue Remover... this is a blended solvent which is safe on most plastics and painted surfaces... we use bucket loads. You can get this from Wessex Chemical Factors down in Dorset.

As far as the Dye goes I've got two ideas.... dump liquid dye and get a tinted self adhesive vinyl film. This will give you predictable tint levels off a roll... and vinyl loves to adhere to acrylic. Grafityp in the midlands are your best bet... they stock the most transparent vinyl colours.

Alternatively you could mix your dye into a pot of Tensol cement and then apply it to the acrylic. Tensol is a two part cement which sets rock hard within an hour.... Anglosol Adhesives would be your best bet for this.

[ImanCarrot]

Thanks Signmaker, I've sorted it by lightly lapping the surface with glass paper then used a big black inky marker (like what you use on white-boards) then cutting the optical surface.

It's amazing how open designers become to compromise when you tell them "yer, I can do it, but it's gona cost £1000" I got the ****ing typical resonse "Oh well, we can probably do without it then". grrr lol

Thanks all!